During the development of functional T lymphocytes, a variety of genes involved in antigen recognition undergo somatic rearrangement. These include the alpha, beta, and gamma chain genes. Recently a fourth rearranging gene, the delta chain gene, embedded in the alpha chain locus, has been described. We have determined the structure of the beta, gamma, and delta chain genes in 15 cases of T-cell acute lymphoblastic leukemia (T-ALL) representing stage I (CD7+, CD1-, CD3-) and stage II (CD7+, CD1+, CD3-) of intrathymic T-cell development. The alpha-delta locus was rearranged in 14 of the 15 cases. In three cases the delta constant region was deleted on both chromosomes, suggesting biallelic V-J alpha rearrangement. A limited pattern of rearrangement of the delta locus was observed in the remaining 11 cases. When the alpha-delta region was rearranged, there was rearrangement of the beta and gamma TcR in all cases except two; in these cases the beta chain was in the germline configuration. These findings support the hypothesis that delta chain gene rearrangement is an early event in T- cell development, possibly contemporary to gamma gene rearrangement, and that the delta locus has a limited repertoire.